Version 4 of the Shared cM Project has been released, utilizing over 60,000 known relationship results submitted by genealogists. The Shared cM Project was begun in 2015 by Blaine Bettinger in order to crowd-source the actual number of shared centiMorgans, cMs, of variously related people who match each through autosomal DNA testing.
Obviously, in order to contribute to the Shared cM Project and participate, you must know how you are related to your matches. You can read about the earlier versions of the project, here.
The Shared cM Project has been very useful for genealogists attempting to determine potential relationships of unknown testers, in particular, because sometimes what we “expect” to see based on academic predictions and models isn’t actually what happens.
Of course, the flip side of that is that sometimes people who contribute relationships don’t understand or report relationships accurately; specifically relationships such as “half,” and “removed.” Nonetheless, with enough data, these reporting errors become statistical outliers. You can participate by contributing your known relationship data through the portal, here.
Blaine’s blog about the new V4 version is here and the full 56-page pdf paper about the results and methodology is here. If you want to understand how the project works, not only is this paper essential reading, it’s a wonderful educational source.
By far, the most common usage of The Shared cM Project results is the interactive tool created at DNAPainter by Jonny Perl.
The Shared cM Project tools are found under the Tools and WATO tab, here.
Click on Shared cM Tool when navigating from the main DNAPainter page.
You’ll see the updated V4 relationship chart, with the field to enter the amount of shared cMs between you and a match above the chart, shown partially above.
Selecting a cM number at random, I entered 1300. The results show the probabilities of various relationships between two people who match at 1300 cMs.
1300 shared cMs can be any of the relationships shown, above. The grey, faded background relationships are not candidates at 1300 cMs, according to V4 of the Shared cM Project.
A new feature added by Jonny provides the ability to click on a relationship and view the histogram from The Shared cM Project showing the submitted relationship amounts. For aunt/uncle at 1300 cMs, 26 people reported that matching amount. The most common amount of shared DNA was 1800 for that relationship category.
You can read Jonny’s latest blog introducing these new features, here.
Thanks to all of the 60,000+ contributors, Blaine and Jonny who made this possible.
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